This invention relates generally to pulse compression systems, and more particularly to means for reducing the range side lobes of the compressed pulse produced in such systems.
It is previously known to transmit pulses from a radar equipment which consist of a carrier modulated according to a certain code pattern. In the radar receiver, a matched filter is provided by means of which a correlation between the incoming radar pulse and the known pattern is carried out. As a result, there is obtained a signal with a well-defined autocorrelation peak surrounded by a number of range side lobes. An example of such a known method to transmit and detect phase-coded radar pulses is described in the U.S. Pat. No. 4,156,876. The above-mentioned method to create a sharp autocrrelation peak in the radar equipment is also known as pulse compression.
The side lobes appearing in the filtered radar pulse are not desirable for the following reason. When the radar is going to detect two different targets A and B, situated at a certain distance from each other, the echo from the target A may interfere with the echo from the target B within a certain mutual distance between the radar targets. This distance is determined by the time interval of the separate radar pulses, and, if the distance is so short that the echo pulse from B appears during the time interval when the echo pulse from the target A appears, interference is obtained. If the target A gives rise to a strong echo while the target B gives rise to a weak echo, the autocorrelation peak in the pulse reflected from B can be completely hidden by the side lobes in the pulse reflected from A. Thus, there is a risk that the target B can not be discovered by the receiver. Therefore, it is important that the side lobes of the reflected pulses be minimized in order to avoid the possibility that the weak target echos will be hidden by the side lobes from an adjacent stronger target echo.
Furthermore, the presence of the range side lobes are disadvantageous as they contribute to the noise level when detecting a target in clutter, that is, unwanted radar echos from ground, sea, rain, etc.
Old methods and apparatus produce autocorrelation function peak-to-maximum side lobe levels less than or equal to 10 times the time-bandwidth product of the signal for large time-bandwidth products unless amplitude weighting is employed. Such weighting, however, reduces the output signal-to-noise ratio. A typical example is a Frank Code compressor as discussed in Cook and Bernfeld, "Radar Signals," 1967, Academic Press. These peak-to-side lobe ratios are not large enough to meet the needs of radars that must range resolve targets whose magnitudes may differ by many orders of magnitude and which must obtain as high a signal-to-noise ratio as possible.